Merge branch 'fix-gcc-arm64-unsigned-chars' into 'develop'

Fix GCC complaining about -1 not being a valid char on arm64 (Linux)

See merge request devel/gdx!72

src/gdlib/gmsstrm.cpp

@@ -82,7 +82,7 @@ constexpr uint8_t signature_header = 0xFF;
 const std::string signature_gams = "*GAMS*"s;
 constexpr int verify_offset = 100;
 
-constexpr static char substChar {/*0x1A*/ std::char_traits<char>::eof()};
+constexpr static char substChar { std::numeric_limits<char>::is_signed ? std::char_traits<char>::eof() : 0x1A };
 
 union TDoubleVar
 {